Many limestone quarries contain some level of metal contaminants, such as iron, aluminum, magnesium, manganese, lead and tin in the calcium carbonate. The metal contaminants are usually in the form of oxides. For example, the presence of iron, in any form, is considered a contaminant in specialty markets, such as food products and pharmaceutical applications. Furthermore, the presence of other metal contaminants in calcium carbonate diminishes the economic value of products, such as paper, plastics, and paints that are produced containing these metal contaminants.
Several physical and chemical processes have been suggested to remove iron from calcium carbonate. Physical removal typically requires grinding the calcium carbonate to such fineness so that the iron can be removed by screening, classification, magnetic separation or floatation. Chemical removal typically requires leaching or bleaching the ground calcium carbonate with solvents followed by filtration. However, these physical and chemical processes for the removal of iron from calcium carbonate are complex, difficult to apply, and the results are erratic and unreliable. There is a need for reliable ways to remove iron and other metal contaminants from calcium carbonate.
The use of electrolytic techniques to remove metal contaminants such as lead and other metals is described in a number of systems. Acar Y. B., "J of Geo-technical Engineering," Vol. 122 No. 3, pps. 175-185, describes an electrolytic technique for the removal of metal contaminants from soil. Dudek F. J.; Morgan, W. A. International Symposium on the Extraction and Applications of Zinc and Lead, Sendai, Japan, May 22-24, 1995, describes an electrolytic technique for reclaiming zinc from steel scrap. Flett D. S.; Covington, J. W.; Winter, D. G.; Spring 1988 Proceedings of the Electrochemical Society Meeting, Volume 88-1, describes a method for removing lead contaminants from zinc solutions. All these techniques describe electrolysis as a method for purifying various systems but none describes using this technique for purifying calcium carbonate.
U.S. Pat. No.5,690,897 describes a method of lowering the iron content of calcium carbonate by using chelating agents. Mahanti, H. S., Barnes, R. M., "Application Spectroscopy," 1983, Vol.37,155.4, and as Golley, C. R. L. and European Patent Application 88304211 describes, purification through the use of floatation. All these techniques provide processes for purifying calcium carbonate. However, none of the techniques make use of the electrolytic process of the present invention.
What has been found to be novel and unanticipated by the prior art is an electrolytic process for reducing the metal contaminants in calcium carbonate, including the reduction of metal contaminants in precursors that are subsequently used in calcium carbonate production.
It is therefore an object of the present invention to provide a process for the removal of metal contaminants from calcium carbonate. Another object of the present invention is to provide a process for the removal of metal contaminants from precursors that are subsequently used in the production of calcium products. Still a further object of the present invention is to provide calcium carbonate products that are free of metal contaminants. Still another object of the present invention is to produce calcium carbonate products that are of sufficient purity such that they find particularly use in market applications where purity of the final product is required.